Devices for regulating the speed of a wheel, also known as a rotor, by a magnetic coupling between a resonator and a magnetic wheel have been known for a number of years in the horological field. A number of patents relating to this field have been granted to Horstmann Clifford Magnetics for the inventions of C. F. Clifford. U.S. Pat. No. 2,946,183 in particular can be cited. The regulating device described in this document has various drawbacks, in particular an anisochronism problem (non-isochronism, in other words a lack of isochronism), specifically a significant variation in the pulsation (angular frequency) of the rotor depending on the torque applied to said rotor. This type of anisochronism results from anisochronism of the oscillator formed by the resonator and the magnetic wheel. The reasons for this anisochronism have already been included in the developments that lead to the present invention. These reasons will become clear later, on reading the description of the invention.
Magnetic escapements with a direct magnetic coupling between a resonator and a wheel formed by a disk are also known from Japanese patent application JPS 5240366 (application no. JP19750116941) and Japanese utility models JPS 5245468U (application no. JP19750132614U) and JPS 5263453U (application no. JP19750149018U). In the first two documents, provision is made to fill rectangular openings of a non-magnetic disk with a powder with high magnetic permeability or a magnetised material. Two adjacent coaxial annular tracks are thus obtained which each comprise rectangular magnetic zones arranged regularly with a given angular period, the zones of the first track being offset or shifted by half a period relative to the zones of the second track. There are therefore magnetic zones distributed alternately on both sides of a circle corresponding to the rest position (position zero) of the magnetic coupling element or component of the resonator. Said coupling element or component is produced by an open loop, depending on circumstances made of a material which is magnetised or has high magnetic permeability, between the ends of which passes the disk, which is driven rotating. The third document describes an alternative in which the magnetic zones of the disk are formed by small individual plates made of material with high magnetic permeability, the magnetic coupling element of the resonator thus being magnetised. The magnetic escapements described in these Japanese documents do not permit significant improvement of the isochronism, in particular for reasons that will be set out below with the aid of FIGS. 1 to 4.
FIG. 1 shows diagrammatically a regulating device or oscillator 2 of the prior art comprising a magnetic escapement of the type described in the above-mentioned Japanese documents. This device comprises a magnetic structure 4 and a resonator 6. The magnetic structure is supported by a mobile 8 made of a non-magnetic material on the surface of which two pluralities of axially magnetised rectangular magnets are arranged, the first and second pluralities of magnets 10 and 12 forming first and second annular magnetic tracks 11 and 13 respectively which are adjacent and concentric. Each of the two pluralities of magnets has the same number of magnets distributed at regular angles and defining the same angular period θP, the first track being shifted by a half period (corresponding to a phase difference of 180°). The resonator 6 is shown symbolically by a spring 15, corresponding to its resilient deformation capacity defined by a resilient constant, and by inertia 16 (symbol ‘I’) defined by its mass and its structure. Said resonator comprises a magnet 18 of rectangular shape and defines an element for coupling to the magnetic structure. Said magnet has an axial magnetisation in the opposite direction to that of the magnets 10 and 12, such that it is arranged in repulsion of said magnets. It is capable of oscillating at a suitable frequency in a resonant mode where it has a radial oscillation relative to the axis of rotation 20 of the mobile 8 which is merged with the central axis of the annular magnetic structure. This resonant mode is excited and maintained when the magnetic structure 4 is driven rotating by a torque in a useful torque range, for example in an anti-clockwise direction at angular frequency ω as shown in FIG. 1. Accordingly, the magnet 18 is situated above the mobile 8 such that its centre of mass is superimposed axially on an intermediate geometric circle defining a common limit or interface of the two concentric and contiguous annular tracks when the resonator is in the rest position.
As the magnets 10 and 12 form zones of magnetic interaction with the magnet 18 of the resonator and are situated alternately on both sides of the above-mentioned intermediate geometric circle, they define a sinuous (sinusoidal) magnetic path with a determined angular period θP, which corresponds to the angular period of each of the first and second annular tracks 11 and 13. When the resonator is magnetically coupled to the magnetic structure driven rotating, the magnet 18 oscillates and follows said sinuous magnetic path and the angular frequency ω of the wheel is defined substantially by the oscillation frequency of the resonator. There is therefore a synchronisation between the frequency of the resonator and the rotational frequency or pulsation of the mobile 8. Here, synchronisation means a determined and constant relationship between two frequencies. The geometric shape of the magnet 18 will be observed, of which the active end portion (shown in FIG. 1) defines a rectangular surface in axial projection in the general geometric plane of the magnetic structure. In other words, said active end portion has a general average outer profile or contour, in a plane parallel to that of the magnetic structure, which is substantially rectangular. In this production of the prior art, the length of said rectangular surface is radial while its width, which is less than its length, is angular relative to the central axis of the annular magnetic structure or tangential relative to the above-mentioned intermediate geometric circle. In the example described here, said length is equal to about twice the width.
FIG. 2 shows diagrammatically, for a portion of the magnetic structure 4 and over a radial range corresponding to the width of the two magnetic tracks 11 and 13, the potential magnetic energy (also known as the potential magnetic interaction energy) of the oscillator 2 which varies angularly and radially. The level curves 22 correspond to different levels of potential magnetic energy. They define equipotential curves. The potential magnetic energy of the oscillator at a given point corresponds to a state of the oscillator when the magnetic coupling element of the resonator is located in a given position (its centre of mass or geometric centre being situated at said given point). It is defined to within a constant. Generally, the potential magnetic energy is defined relative to a reference energy which corresponds to the minimal potential energy of the oscillator. In the absence of any dissipative force, said potential energy corresponds to the work required to take the magnet from a position of minimal energy to a given position. In the case of the oscillator in question, said work is supplied by the torque applied to the mobile 8. The potential energy accumulated in the oscillator is transferred to the resonator when the coupling component of the resonator returns to a position of lower potential energy, in particular of minimal potential energy, by a radial movement relative to the axis of rotation of the mobile (in other words depending on the degree of freedom of the useful resonant mode). In the absence of any dissipative force, this potential energy is transformed into kinetic energy and resilient energy in the resonator by the work of the magnetic force between the coupling element of the resonator and the magnetic structure. Thus the torque supplied to the wheel serves to maintain the oscillation of the resonator which in return applies a braking force to the wheel regulating its angular frequency.
The outer annular track 11 defines an alternation of zones of low potential energy 24 and zones of high potential energy 26 whereas the inner annular track 13 defines, with an angular phase difference of half an angular period θP/2 relative to the first track (in other words a phase difference of) 180°, an alternation of zones of low potential energy 28 and zones of high potential energy 30. The line 32 gives the position of the centre of the magnet 18 when the oscillator 2 is excited and the mobile 8 is therefore driven rotating with a determined torque. Said line illustrates the oscillation of the magnet of the resonator 6 in a system of reference linked to the mobile. As said magnet is in repulsion of the magnets of the magnetic structure 4, the zones of low potential energy correspond to the zones between the magnets of the magnetic structure whereas the zones of high potential energy correspond to the zones of said magnets, in other words to situations where the magnet 18 is at least in part superimposed on the magnets of the magnetic structure. It will be noted that in the case where the magnets are arranged in attraction, or alternatively in the case where the magnetic structure or the coupling component of the resonator is made of a ferromagnetic material, there is a spatial reversal between the zones of low potential energy and the zones of high potential energy compared with the case where the magnets are in repulsion.
Observing the level curves 22 of potential magnetic energy and oscillation 32, it will be seen that the oscillator accumulates potential magnetic energy at each alternation of the oscillation basically when the magnet 18 has reached its maximum amplitude and begins to return to its zero position. It can also be seen that the potential energy of the oscillator diminishes over a large part of each alternation. The force F applied to the magnet of the resonator is given by the potential magnetic energy gradient, which is perpendicular to the level curves 22. The angular component (degree of freedom of the magnetic structure) works by reaction on the wheel whereas the radial component (degree of freedom of the resonator) works on the coupling component of the resonator. The angular force corresponds on average to a braking force of the mobile because the angular reaction force is for the most part opposed to the direction of rotation of said mobile over a period of oscillation. The radial force corresponds to a thrust force on the oscillating structure of the resonator. It can be seen that the force F (see FIG. 2) has a radial component over a significant distance between the oscillation extrema 32. A thrust force therefore acts on the magnet of the resonator in the majority of each alternation.
If the potential energy curves 22 are analysed and the behaviour of the oscillator in question is studied in this case in relation to the torque applied to the wheel, at least two major drawbacks of such a regulating device can be observed. Firstly, the range of values for the torque is small and secondly the regulating device has significant anisochronism. Said anisochronism is so great in the prior art that it is not possible to produce a horological movement that has a suitable operating range, in other words with acceptable precision.